Sailboats need a means of moving when the wind is not blowing. Whether a sailor finds himself at the far end of a lake when the wind dies, or enters a sheltered harbor with little or no wind, he would welcome an easy way to move. On large boats this is accomplished by using an engine, but on small boats an engine is not often feasible. Using an oar or paddle is an option, but may be awkward and requires shifting from side to side. Sculling with the rudder can produce some forward motion but is not very efficient, since the rudder is designed for steering and is not configured to provide much forward thrust.